A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake system of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, comprises a solenoid actuator that is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, evaporative emission space that is cooperatively defined primarily by the tank headspace and the canister is purged to the engine intake system through the canister purge valve. For example, fuel vapors may be purged to an intake manifold of an engine intake system by the opening of a CPS-type valve in response to a signal from the engine management computer, causing the valve to open in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace, and/or stored in the canister, for entrainment with combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure, and then detecting a change in that substantially different pressure which is indicative of a leak.
It is believed fair to say that from a historical viewpoint two basic types of vapor leak detection systems for determining integrity of an evaporative emission space have evolved: a positive pressure system that performs a test by positively pressurizing an evaporative emission space; and a negative pressure (i.e. vacuum) system that performs a test by negatively pressurizing (i.e. drawing vacuum in) an evaporative emission space. The former may utilize a pressurizing device, such as a pump, for pressurizing the evaporative emission space; the latter may utilize either a devoted device, such as a vacuum pump, or engine manifold vacuum created by running of the engine.
Commonly owned U.S. Patents and Patent Applications disclose various systems, devices, modules, and methods for performing evaporative emission leak detection tests by positive and negative pressurization of the evaporative emission space being tested.